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Tribology question

Tribology question

Tribology question

I have a problem with a paste that I am attempting to extrude through a glass syringe. The paste is about the consistency of toothpaste and has very small (under 150 micron) particles suspended in it. When I attempt to apply a constant load to the teflon pusher of a glass syringe, I do not get a constant flow rate. When I lubricate the system with a PFPE lubricant (which is not soluble in the paste or any of its components) it seems to get a little better, but I still don't get a constant flow rate. Does anyone have any experience with extruding pastes and or tribology that can help me take this problem apart. One interesting thing that I noticed is that the paste seems to stick very well to stainless steel, so I'm wondering if it is sticking to the glass as well, causing the system to bind.

RE: Tribology question

When you apply a constant load to the teflon pusher of the glass syringe, does it run smoothly or does it stuck sometimes? If it runs smoothly, maybe when you admit that paste onto the glass syringe you are also getting air which may create air bubbles inside the syringe leading to a non constant flow rate. Have you checked if you are also expelling air?

RE: Tribology question

Thanks for the tip. I've used a centrifuge to remove the entrapped air, so I'm pretty sure there isn't any air in it. When I apply the constant load it is difficult to see what's happening since I'm targeting a flow rate that would move the plunger approximately 0.02mm/min.

RE: Tribology question

What is the glass load in the mixture? You may have created a thixotropic material. Too much glass most likely makes the material not readily extrudable. Glass on glass does not work well under pressure.

RE: Tribology question

RE: Tribology question

I think there is some confusion. The particles are not glass beads. It isn't possible to test the material without the particles since the particles are in suspension in a very high concentration and it would give very different results. I have tested a high viscosity silicone oil (20,000 cP) and was able to obtain repeatable results.

RE: Tribology question

"particles since the particles are in suspension in a very high concentration and it would give very different results. I have tested a high viscosity silicone oil (20,000 cP) and was able to obtain repeatable results."

I think that was essentially my point. Your "particles" are relatively large, and you seem to be saying that the particle loading is high, both of which are not conducive to smooth flow. You should at least try a run with your mixture with a lower load, or a wider orifice, and see if that changes things.

What is the detail of the transistion connecting the "opening" and the end of the syringe?
Is the "opening" part of a separate component screwed on to the end of the syringe?
Are there any shoulders or steps in the transition?

Depending what you are starting with, Perhaps a gentler smoother profile would help buy some consistency.

For gravity fed systems the transition is very important to prevent a variety of problems.
"first in, first out, bridging, rat-holing"

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